This invention relates to pi-conjugated (or π-conjugated) organoboron polymers and their use in thin film electronic devices. The invention further relates to methods for the fabrication of such devices as well as to methods of preparation of certain pi-conjugated organoboron polymers having improved solubility and stability. Pi-conjugated organoboron polymers are of interest because of their opto-electronic properties and are useful in thin-film plastic electronic devices, such as organic thin film transistors (TFTs), organic light emitting diodes (OLEDs), printable circuits, organic supercapacitors and organic photovoltaic (OPV) devices
The specific functions of many electronic components and devices arise from the unique interactions existing between p-type and n-type conducting and semiconducting materials. Until a few years ago, inorganic conductors and semiconductors entirely dominated the electronic industry. In recent years there has been a major worldwide research effort to develop conducting and semiconducting organic compounds and polymers, and to use them to fabricate plastic electronic devices, such as organic thin film transistors (TFTs), organic light emitting diodes (OLEDs), printable circuits, organic supercapacitors and organic photovoltaic devices. Plastic electronic components offer several potential advantages over traditional devices made of inorganic materials; they are flexible and can be manufactured by inexpensive ink-jet printing or roll-to-roll coating technologies.
Intrinsically conducting polymers (ICPs) are polymers with extended π conjugation along the molecular backbone, and their conductivity can be changed by several orders of magnitude by doping. P-doping is the partial oxidation of the polymer by a chemical oxidant or an electrode which causes depopulation of the bonding π orbital (HOMO) with the injection of “holes”. N-doping is the partial reduction of the polymer by a chemical reducing agent or electrode with the injection of electrons in the antibonding π system (LUMO, MacDiarmid A., Angew. Chem. Int. Ed., 40, 2581-2590, 2001). The doping process incorporates charge carriers (either electrons or holes) into the polymer backbone, and as a result the polymer becomes electrically conducting to a level that is commensurate with its doping level.
An equally important class of electronic polymers is the conjugated semiconducting polymers. These polymers, like ICPs, are able to support the injection of p-type or n-type charge carriers; however the charge carriers are often few in number and are transient species that ultimately decay or are transferred to a different material. Some ICPs function as semiconducting polymers in their undoped state, however other ICPs are not stable in their semiconducting state. Electrical conductivity and work function are the key parameters for characterizing ICPs, while charge carrier density and mobility, and energy levels are the key parameters for characterizing semiconducting polymers. One special class of semiconducting polymers is the group of light emitting polymers. These polymers are also able to support the injection and transport of both positive and negative charges (although one carrier is often preferred). When holes and electrons recombine within the electroluminescent material, a neutral excited species (termed an exciton) forms that decays to the ground state, liberating energy in the form of light (Salaneck, W. R. et al., Nature, 397, 121-128, 1999).
Thus, certain pi-conjugated polymers are well known to possess semiconducting properties which are due to the formation of interconnected molecular orbitals along the pi-bonding and pi-antibonding structure of the conjugated backbone. Charges that are introduced onto such polymer chains (either by addition or removal of an electron) are free to travel over a certain distance along the polymer chain giving rise to semiconducting properties. The number of charges that can be injected, the ease of introducing charges, the distance that a charge can travel (mobility), and the type of charge (positive holes or negative electrons) that is more favorable depend upon the electronic properties of the polymer. By careful design of the structure, a polymer that favors electrons (called an n-type semiconductor) or holes (called a p-type semiconductor) as the dominant form of charge carrier can be selected. This is done through introduction into the polymer of selected atoms, organic groups and/or substituent groups based on their electron-donating or electron-withdrawing properties. Appropriate selections of atoms or chemical groups for introduction into the polymer structure lead to a conjugated polymer structure that is either electron-rich or electron-deficient. Electron-rich polymer structures have p-type semiconducting properties and electron-deficient polymer structures have n-type semiconducting properties.
Most pi-conjugated hydrocarbon polymers such as polyacetylene, poly(phenylenevinylene), poly(paraphenylene), polyfluorenes and their derivatives readily support the injection of both electrons and holes (the respective n-type and p-type charge carriers). In fact, theoretical calculations show that for certain hydrocarbon conjugated polymers such as poly(paraphenylene) there is a perfect electron-hole symmetry [i.e. the frontier orbitals of positively and negatively charged carriers are fully symmetrical], indicating that electron and hole conduction are equally favorable processes (Kertesz, M. in Handbook of Organic Conductive Molecules and Polymers, Vol. 4, Ed. Hari Singh Nalwa, J. Wiley & Sons, Chichester, UK, p. 163, 1997). This symmetry can be broken by introducing atoms other than carbon (heteroatoms, i.e., O, N, S, etc.), organic groups and/or substituent groups that are either electron-rich or electron-deficient, thus favoring either the injection of holes or electrons, respectively. It is generally easier to design electron-rich conjugated polymers than electron-deficient conjugated polymers. An electron-rich polymer can be created by appropriate introduction of an electro-negative heteroatom, such as sulfur, nitrogen or oxygen into the conjugated polymer. A variety of chemistries are available in the art for introducing electro-negative heteroatoms into such polymers. As a result a large number of p-type conducting polymers have been developed and characterized over the past two decades. Furthermore, many p-type conducting and semiconducting polymers have been used in commercial devices and are successfully competing with conventional inorganic semiconductors and conductors.
In contrast, it is more difficult to design electron-deficient conjugated polymer systems. Most of the polymers currently used as n-type semiconductors are hydrocarbon-based polymers [especially poly(phenylenevinylene)] carrying electron-withdrawing substituents such as cyano or nitro groups (Friend, R. H. et al. Nature, 395, 257-259, 1998; Holmes et al. Angew. Chemie. In. Ed., 37, 402-428, 1998), polymers containing oxadiazole, quinoxaline, or pyridine units (Bradley, et al. Appl. Phys. Lett., 69, 881-883, 1996; Holmes et al. Angew. Chemie. In. Ed., 37, 402-428, 1998; Andersson et al. Macromolecules, 35, 1638-1643, 2002), and a few ladder polymers such as BBL ({poly(7-oxo,10H-benz[de]imidazo[4′,5:5,6]-benzimidazo[2,1-a]isoquinoline-3,4:10,11-tetrayl)-10-carbonyl}) (Sherf, U. “Conjugated Ladder-Type Structures,” in Handbook of Conducting Polymers, 2nd Ed.”. Ed. T. A. Skotheim, R L Elsenbauer, J. R. Reynolds, Marcel Dekker, New York, 363-379, 1998). Unfortunately, current n-type semiconducting polymers have generally poor properties, including low charge carrier density and low carrier mobility. Furthermore, most of these materials are difficult to process, and some of them are difficult to synthesize.
In some cases, n-type semiconducting non-polymeric species, such as functionalized fullerenes, molecular glasses and metal complexes, are used instead of polymers (Strohriegl, P. et. Al, Advanced Materials, 14, 1439-1451, 2002; Shaheen, S. et. al., Appl. Phys. Lett., 78, 841-843, 2001). The disadvantage of these non-polymeric semiconducting species is the low charge carrier mobility due to the limited conjugation (due to low molecular weight), and the fact that they often need to be processed by vacuum deposition techniques. Thus, there is a significant need in the art for new n-type conducting and semiconducting materials having improved charge carrier mobility, which are more readily synthesized and processed. The present invention provides n-type semiconducting polymers which provide such improvements.
There are two basic ways to make a pi-conjugated polymer structure that is electron deficient. First, as noted above, the conjugated backbone of the polymer can be chemically modified by substitution with electron withdrawing substituent groups, such as cyano or nitro groups. Such pendant modification is effective to impart some electron deficiency to the pi-conjugated polymer. For example, poly(para-phenylene vinylene) has been modified with cyano and other pendant groups to produce a pi-conjugated semiconducting polymer with n-type properties (Granstrom et al. Nature 395, 257-260, 1998). A second and more effective way to impart n-type semiconducting properties is to directly modify the backbone of the polymer with electron deficient atoms or organic structures. Holmes et al. prepared pi-conjugated oxadiazole-containing polymers that exhibited n-type semiconducting properties and photoluminescence (Li et al. J. Chem. Soc. Chem. Commun. 2211-2212, 1995). Yamamoto et al. prepared pi-conjugated quinoxaline-containing polymers that also exhibited n-type semiconducting properties, photoluminescence, and electroluminescence. Both the oxadiazole and quinoxaline structures are known to impart electron deficiency in molecules. Similarly, Babel and Jenekhe. prepared pi-conjugated polymers incorporating regioregular dioctylbithiophene and bis(phenylquinoline) units in the backbone of the polymer and demonstrated both PLED (polymer light-emitting diodes) and OFET (organic field-effect transistors) prototype devices utilizing these materials (Babel, A., Jenekhe, S. A. Adv. Mater., 14, 371-374, 2002).
Certain non-polymeric, pi-conjugated, organoboron molecules have been observed to be electron deficient (Noda et al. J. Am. Chem. Soc. 120, 9714-9715, 1998; Matsumi et al. Polymer Bulletin 50, 259-264, 2003). This is due to the valence electronic structure of the boron atom and its ability to form multiple stable bonds with carbon atoms. The empty p-orbital of boron can join in the pi-conjugated system without any added electron density (Zweifel et al. J. Organomet. Chem. 117, 303-312, 1976). The possibility of delocalization of pi electrons between the vacant p orbital of boron and the pi orbitals of conjugated organic substituents has been extensively studied on mono- and di-vinylhaloboranes and trivinylborane. These molecules exist only in a planar conformation, suggesting that there is, in fact, delocalization of the vinyl pi electrons over the boron atom (Pelter, A., and Smith, K. “Triorganylboranes,” in Comprehensive Organometallic Chemistry, Vol 3, 792-795, 1979). Theoretical calculations performed with the LCAO and self-consistent field methods (Good, C. D., and Ritter, D. M. J. Am. Chem. Soc., 84, 1162-1165, 1962) as well as 13C-NMR studies (Yamamoto, Y. and Moritani, I. J. Org. Chem., 40, 3434-3437, 1975) also predict considerable delocalization of the vinyl pi electrons over the carbon-boron bonds.
Marder et al. report that three-coordinate boron species are equivalent to carbonium ions, and are thus extremely electron-deficient systems. However, if the boron is sterically protected, for example, with bulky trimethylphenyl groups, the resultant materials are air-stable (Marder et al. J. of Solid State Chemistry, 154, 5-12, 2000). Kaim and co-workers report that low molecular weight, non-polymeric, pi-conjugated organoboron compounds having redox properties that are analogous to nitrogen-containing pi-conjugated molecules. In fact, under chemical or electrochemical reduction, organoboron compounds form a series of anions of the type: —BR2, —BR2.−, ═BR2−, while nitrogen-containing compounds upon oxidation form the series of cations: —NR2, —NR2.+, ═NR2+ (Fiedler et al. Inorg. Chem., 35, 3039-3043, 1996). This indicates that pi-conjugated organoboron compounds are redox active and are effectively easy to reduce. The use of certain organoboron, non-polymeric pi-conjugated molecules as an electron transport layer (ETL) in molecular organic light-emitting diodes is reported by Shirota and Noda. These authors report an improvement in maximum luminescence by a factor of 1.6 to 1.8 compared to an identical single layer device that does not contain the organoboron ETL (Shirota Y. and T. Noda J. Am. Chem. Soc., 120, 9714-9715, 1998). The organoboron ETL materials of Shirota and Noda are non-polymeric molecules of defined structure having a specific molecular weight and are not pi-conjugated organoboron polymers.
Chujo and co-workers reported a number of pi-conjugated, organoboron polymers wherein the boron atoms of the polymer backbone are substituted with bulky aromatic groups such as 2,4,6-trimethylphenyl(mesityl) or 2,4,6-triisopropylphenyl(tripyl) (Matsumi et al. J. Am. Chem. Soc., 120, 10776-10777, 1998; Matsumi et al. J. Am. Chem. Soc., 120, 5112-5113, 1998; Matsumi et al. Macromolecules, 32, 4467-4469, 1999; Matsumi et al. Polymer Bulletin, 44, 431-436, 2000, Chujo et al. Polymer, 41, 5047-5051, 2000). The authors concluded that the bulky protecting groups on boron led to stable non-conjugated polymers with weight average molecular weights that remained stable with constant exposure to air for two weeks (Chujo et al. Polymer 41, 5047-5051, 2000). Chujo and coworkers also prepared pi-conjugated, organoboron polymers wherein the boron atoms of the polymer backbone are substituted with phenyl groups (Miyata et al. Polymer Bulletin, 42, 505-510, 1999), or bonded to a quinolate moiety (Nagata et al. Macromolecules, 40, 6-8, 2007.) These polymers have absorption maxima in the visible region and are highly fluorescent when irradiated with UV light, suggesting the existence of an extended π-conjugation across the boron atoms. The polymers are also soluble in common organic solvents and stable in air and moisture in the pristine (undoped) state. Chujo and co-workers have also reported the n-doping of a pi-conjugated, organoboron polymer with triethylamine to a conductivity of 10−6 S/cm (Kobayashi et al. Synthetic Metals, 135-136, 393-394, 2003). The n-type semiconducting properties and photoluminescence of these materials have been reported, but the materials were not shown to be useful in thin film, organic polymer electronic devices, such as OPVs (organic photovoltaics), PLEDs, or OFETs.
Jäkle reported boron-modified polythiophenes for use in chemical sensors wherein the boron atoms of the polymer backbone are substituted with 4-isopropylphenyl, pentafluorophenyl, and ferrocenyl groups (Sundararraman, et al., J. Am. Chem. Soc., 127, 13748-13749, 2005.) Siebert and co-workers reported the synthesis of certain pi-conjugated organoboron polymers containing thiophene units by a hydroboration polymerization (Corriu et al., Chem. Commun. 963-964 1998).
U.S. Pat. Nos. 3,269,992, 3,203,909, 3,203,930, 3,203,929, 3,166,522, and 3,109,031 report the preparation of certain non-conjugated organoboron polymers. U.S. Pat. No. 6,025,453 reports polymers containing at least an alkynyl group, at least one silyl group and at least one boranyl group and their use for making high temperature oxidatively stable thermosetting plastics.
Kanitz et al. report non-conjugated polymeric perarylated borane copolymers and their uses (published US Patent application 2006/0229431 A1; PCT Application WO 2005/063919 and WO 2006/0229431). Cunningham et al. (U.S. Pat. No. 6,057,078) report certain polyborane and polyborate photoinitiators.